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Maybe have a working stub now.

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David Given 2022-08-09 23:32:41 +02:00
parent 21b30ccadb
commit b48b5b13ce
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plat/msdos386/stub.s Normal file
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#
! $Source$
! $State$
! $Revision$
! Declare segments (the order is important).
.sect .text
.sect .rom
.sect .data
.sect .bss
.sect .text
.use16
exe_header:
.data2 0x5a4d ! magic number
.data2 exe_last_page ! number of bytes in last loadable page
.data2 exe_text_pages ! size of .exe, in pages
.data2 0 ! number of relocation entries
.data2 0 ! start of loadable area, in 16-byte paragraphs
.data2 exe_ram_paras ! required RAM size, in 16-byte paragraphs
.data2 exe_ram_paras ! maximum RAM siz, in 16-byte paragraphs
.data2 0 ! initial SS, relative to program
.data2 stack ! initial SP
.data2 0 ! checksum (ignored)
.data2 exe_start ! initial IP
.data2 0 ! initial CS, relative to program
.data2 0 ! offset of relocation table
.data2 0 ! overlay number
exe_start:
! On entry, DS=ES=PSP. Make DS=CS, so we're running in tiny mode.
push cs
pop ds
mov (rseg), ds
mov (pspseg), es
! Ensure that at least two handles are available by blindly closing some.
mov bx, 19 ! close handle
movb ah, 0x3e
int 0x21
mov bx, 18 ! close handle
movb ah, 0x3e
int 0x21
! Scan the environment to find the program name.
eseg mov es, (0x2c) ! environment pointer segment from PSP
xor di, di
xorb al, al
mov cx, 0xffff
1:
repne scasb ! find end of next string
eseg cmpb (di), 0
jnz 1b
2:
add di, 3
mov si, di
repne scasb
! es:si now points at the program name, and es:di at one past the end.
mov cx, di
sub cx, si
sub sp, cx ! allocate space on stack
mov di, sp ! es:si source, ds:di dest
1:
eseg movb al, (si)
inc si
movb (di), al
inc di
cmpb al, 0
jnz 1b
! Open the file. (0x5f)
mov dx, sp
mov ax, 0x3d00
int 0x21
jc no_file
mov (fh), ax
add sp, cx ! don't need filename any more
! Get the file length (== text len).
mov bx, ax
mov ax, 0x4202
xor cx, cx ! high offset
xor dx, dx ! low offset
int 0x21 ! lseek
mov (textlen+0), ax
mov (textlen+2), dx
! Initialise DPMI.
mov ax, 0x1687
int 0x2f
or ax, ax
jnz no_dpmi
mov (pmode_switch+0), di ! write back PMODE switch routine
mov (pmode_switch+2), es
or si, si ! do we need a DPMI private area?
jz 1f
mov bx, si
movb ah, 0x48
int 0x21 ! allocate memory from DOS
mov es, ax ! data area segment -> es
1:
! Switch to protected mode.
mov ax, 1 ! 32-bit app
callf (pmode_switch)
jc bad_dpmi
! We're now in protected mode. (0xa9)
mov (psegcs), cs
mov (psegds), ds
! Allocate space for the code segment.
mov ax, 0x0000
mov cx, 1
int 0x31 ! allocate LDT
jc bad_dpmi
mov es, ax
mov cx, (textlen+0)
mov bx, (textlen+2)
mov ax, 0x0501
int 0x31 ! allocate linear address
jc bad_dpmi
mov (pmemhandle+0), si
mov (pmemhandle+2), di
mov dx, cx
mov cx, bx
mov bx, es
mov ax, 0x0007
int 0x31 ! set segment base address
jc bad_dpmi
mov bx, es
mov dx, (textlen+0)
mov cx, (textlen+2)
mov ax, 0x0008
int 0x31 ! set segment limit
mov cx, cs
and cx, 3
shl cx, 5
or cx, 0xc09b ! 32-bit, big, code, non-conforming, readable
mov bx, es
mov ax, 0x0009
int 0x31 ! set descriptor access rights
! Load the program. (0xff)
mov bx, (fh)
mov ax, 0x4200
xor cx, cx ! high offset
xor dx, dx ! low offset
int 0x21 ! lseek
o32 xor edi, edi ! destination 32-bit register
1:
movb ah, 0x3f
mov bx, (fh)
mov cx, TRANSFER_BUFFER_SIZE
mov dx, transfer_buffer
o32 push edi
call int21 ! read up to 32kB into transfer buffer
o32 pop edi
cmp ax, 0
je 2f
o32 movzx ecx, ax ! number of bytes read
o32 mov esi, transfer_buffer
o32 rep movsb
jmp 1b
2:
! Close the file.
movb ah, 0x3e
mov bx, (fh)
int 0x21 ! close
! Jump to the new segment and enter 32-bit mode!
o32 mov eax, (pmemhandle)
o32 movzx ebx, (rseg)
push es
push text_top
retf
bad_dpmi:
mov si, bad_dpmi_msg
jmp exit_with_error
no_file:
mov si, no_file_msg
jmp exit_with_error
no_dpmi:
mov si, no_dpmi_msg
! fall through
! Displays the message in si and exits.
! This uses a loop because from protected mode any interrupt which
! takes a pointer parameter requires special treatment.
exit_with_error:
movb dl, (si)
cmpb dl, 0
je 1f
inc si
movb ah, 2
int 0x21 ! print character
jmp exit_with_error
1:
mov ax, 0x4cff
int 0x21 ! terminate with error code al
pmode:
.use32
mov ax, 0x4c00
int 0x21
.use16
! Simulate DOS interrupt.
int21:
mov (dpmi_eax), ax
mov (dpmi_ebx), bx
mov (dpmi_ecx), cx
mov (dpmi_edx), dx
mov ax, (rseg)
mov (dpmi_ds), ax
push es
push ds
xor ax, ax
mov (dpmi_ss), ax ! zero stack: DPMI host allocates one.
mov (dpmi_sp), ax
push ds
pop es
mov di, dpmi_edi
mov ax, 0x300
mov bx, 0x21
int 0x31 ! simulate DOS interrupt
pop ds
pop es
mov ax, (dpmi_eax)
mov bx, (dpmi_ebx)
mov cx, (dpmi_ecx)
mov dx, (dpmi_edx)
push (dpmi_flags)
popf
ret
bad_dpmi_msg:
.asciz "DPMI error during setup"
no_file_msg:
.asciz "Couldn't open .exe"
no_dpmi_msg:
.asciz "No DPMI host installed"
text_top:
exe_text_pages = [text_top - exe_header + 511] / 512
exe_last_page = [text_top - exe_header] % 512
.sect .rom
.sect .bss
bss_start:
dpmi_edi: .space 4
dpmi_esi: .space 4
dpmi_ebp: .space 4
.space 4 ! reserved
dpmi_ebx: .space 4
dpmi_edx: .space 4
dpmi_ecx: .space 4
dpmi_eax: .space 4
dpmi_flags: .space 2
dpmi_es: .space 2
dpmi_ds: .space 2
dpmi_fs: .space 2
dpmi_gs: .space 2
dpmi_ip: .space 2
dpmi_cs: .space 2
dpmi_sp: .space 2
dpmi_ss: .space 2
pmode_switch: .space 4
rseg: .space 2 ! real mode
pspseg: .space 2 ! real mode
psegcs: .space 2 ! protected mode 16-bit code segment
psegds: .space 2 ! protected mode 16-bit data segment
pmemhandle: .space 4 ! protected mode linear memory handle
fh: .space 2
textlen: .space 4
.space 128
stack:
TRANSFER_BUFFER_SIZE = 32*1024
transfer_buffer:
.space TRANSFER_BUFFER_SIZE
bss_top:
exe_ram_paras = [bss_top - bss_start + 15] / 16
! vim: ts=4 sw=4 et ft=asm